Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH(s)”) have been widely used as a material that is moldable through melting and superior in gas barrier properties. For example, EVOHs have been used as a material for films and sheets to be formed by melt molding. EVOH layers constituted with the sheet and the like have been used as a packaging material after being laminated on a thermoplastic resin layer containing a polyolefin-derived resin, etc. as a principal component. Such a packaging material including the EVOH layer may be thermoformed and thereafter utilized as a packaging container. Since the packaging container is superior in oxygen barrier properties due to including the EVOH layer, the packaging container has been widely used in intended usages in which superior oxygen barrier properties are demanded, for example, in fields of foods, cosmetics, medical drugs, chemicals, and toiletries. In this regard, remnants, defective products and/or the like generated in the production of the various types of molded articles may be recovered, subjected to melt molding, and recycled as at least one layer of a multilayer structure including a polyolefin layer and an EVOH layer. Such a recovery technique is useful and extensively adopted in light of a waste reduction and an economical efficiency.
However, in the recycling of the recovered material of the multilayer structure including the polyolefin layer and the EVOH layer, gelation that occurs due to heat deterioration of the material during melt molding, and adhesion of a deteriorated matter to the inside wall of an extruder have made continuous melt molding over a long time period difficult. In addition, such an adhesion of the deteriorated matter to the inside wall of the extruder is disadvantageous in that the roughness of the surface of the resulting molded article is likely to generated. Furthermore, these drawbacks become significant with the repetition of the recycling of the recovered material from the multilayer structure.
To address such drawbacks, for example, Patent Document 1 discloses a resin composition that contains: a polyolefin; an ethylene-vinyl acetate copolymer saponified product having an ethylene percentage content of 20 to 65 mol % and a degree of saponification of a vinyl acetate component of 96 mol % or greater; at least one compound selected from a higher fatty acid metal salt having 8 to 22 carbon atoms, an ethylenediaminetetraacetic acid metal salt and hydrotalcite; and an ethylene-vinyl acetate copolymer saponified product having an ethylene percentage content of 68 to 98 mol % and a degree of saponification of a vinyl acetate component of 20% or greater. Furthermore, it is disclosed that salts of higher fatty acids such as lauric acid, stearic acid and myristic acid with a metal such as calcium, magnesium or zinc are suitable as the higher fatty acid metal salt having 8 to 22 carbon atoms. This resin composition reportedly has superior compatibility, and the molded article obtained by using the resin composition reportedly has no wave pattern on the surface thereof, leading to a superior appearance. However, the resin composition disclosed in Examples of Patent Document 1 and containing calcium stearate may generate a substance adhering to a screw, and as a result, the resulting molded article may have a rough surface.
Patent Document 2 discloses a production method of a fuel container, characterized in that the fuel container has a reground layer (recovery layer) containing ground matter of a laminate, and the laminate has: an intermediate layer that contains an ethylene-vinyl acetate copolymer saponified product having an ethylene content of 10 to 60 mol % and a degree of saponification of a vinyl acetate component of 95 mol % or greater and containing at least one selected from magnesium, calcium and zinc in a proportion of 10 to 500 ppm; and a thermoplastic resin laminated at least on both sides as the outermost layer. In this method, the intermediate layer of the laminate used for the reground layer contains an ethylene-vinyl acetate copolymer saponified product with which a fatty acid metal salt is blended. At least one selected from a magnesium salt, a calcium salt and a zinc salt is used as the fatty acid metal salt. Moreover, a lower fatty acid and/or a higher fatty acid are/is used as the fatty acid. According to the production method, a fuel container that is superior in melt formability, mechanical characteristics and the like is reportedly obtained. However, when zinc stearate is blended in an amount as disclosed in Examples of Patent Document 2, there still remain drawbacks that deteriorated matter is likely to adhere to a screw during the melt molding of the laminate, and the resulting molded article is likely to have a rough surface.
Furthermore, the techniques disclosed in the documents described above are still unsatisfactory in light of molding defects generated in the case of multiple recycling of the recovered material as well as the inhibition of the deterioration of the impact resistance caused by the molding defects.
In addition, a production method of EVOH has been known in which crotonaldehyde is coexisted in addition to ethylene and vinyl acetate in a polymerization step to produce the EVOH (see Patent Document 3). According to this production method, an adhesion of scales in the interior of a polymerization tank can be inhibited by the coexistence of crotonaldehyde during the polymerization. As a result, films of the EVOH produced by this production method, generation of fish eyes resulting from the scales detached and mixed into the polymer can be reportedly decreased.
However, crotonaldehyde added in the polymerization is partially consumed in the polymerization step and a saponification step. In addition, crotonaldehyde has a solubility in water of as high as 18.1 g/100 g (20° C.) (The MERCK INDEX 14th 2006). On the other hand, the production method of EVOH typically includes the step of washing away sodium acetate produced in the neutralization after the saponification with water. Thus, crotonaldehyde added in the polymerization is substantially completely eliminated in the washing step during the production of EVOH, and thus hardly remains in a product such as an EVOH film. Therefore, according to the production method described above, the effects of the addition of the unsaturated aldehyde on an improvement of thermal stability and properties for operation for a long time period (long-run workability) in the thermoforming, and the like are unknown.
As described above, according to the conventional resin compositions and production methods, it is difficult to improve an unfavorable appearance and inhibit the deterioration of the impact resistance when the recovery and recycling of the resin compositions is repeated multiple times.